My 99 F-150 makes a loud scraping noise when I turn on the heat or a/c. It only happens when the truck has been sitting for a few days and I've took it to the dealer many times but the truck will not make the noise. Does anyone know what is wrong or is this normal?

 

Well, it doesn't sound normal to me.

Maybe you should try to determine where the sound is coming from. Is it inside the cab? If so, then it's probably the blower. If it's outside, it could be the compressor. You'll have to do some investigating on your own since it's an intermittent problem.

 

Thanks for your help.
I'm almost positive it comes from under the truck, but it is loud enough to here in the cab. Would the computer realize there is a problem or would it only see it if the part was broken?